Bash shell

Guide on the usage of the most common commands of the bash shell

Generic Linux Command Syntax

command [-options] [arguments]
  • If a command is to long to stay on a single row, it may be continued on the next row using the character ”\” as last character of the row
  • More than one command can be executed on the same line, separating the commands with ”;”
command1; command2;...
  • In this case commands will be executed sequentially

Help

All linux commands are documented, and their documentation can be accessed using:

  • man <command>: gives an extensive description of command
  • apropos <command>: search a keyword in the manual page descriptions
  • whatis <command>: gives a short description of command
  • command -h or command -help: gives a description of command options

The Unix file system

The Unix file system is a hierarchical file system organized in directory:

  • / is the symbol that represents the root of the file system
  • . (dot) is the actual directory
  • .. (dot dot) is the father directory
  • ~ is the home directory, for example /home/username

A file that starts with ”.” (dot) is an hidden file.

Files within a directory can be accessed using:

  • an absolute path: /dir1/dir2/file
  • a relative path: subdir1/subdir2/file

Files Commands

cp [-fir] src1 src2 ... dest
  • copy one or more files, possibly in a directory
rm [-fir] file1 file2 ...
  • delete file1, file2, …
  • files can be directories if the -r option is used. In this case the entire directory content will be recursively deleted
mv [-fi] file1 file2 ... dest
  • move one or more file, possibly in a directory

File commands options are:

  • -f force, no confirmation is required
  • -i: interactive, confirmation is required for each file
  • -r: recursive, the file command works recursively for each subdirectory

Directories Commands

cd dir
  • goes into the dir directory
  • cd ..: goes into the father directory
  • cd (alone): goes into the home directory
pwd
  • prints the name of the current directory
mkdir dir
  • makes the dir directory
rmdir dir
  • removes the dir directory
  • dir must be empty

Link Commands

A Symbolic link is a file that contains a reference to another file or directory

  • Hard link: more than one file name point to the same file
  • Soft link: is a special file that contains the file that must be pointed
ln src alias
  • makes an hard link between src and alias
  • hard links may not normally point to directories and they can not link paths on different volumes, for this reason usually soft link are preferred
ln -s src alias
  • makes a symbolic soft link between src and alias

Files and Directories protection

In Linux, every file or directory has special protections that allow differential access to the different users of the system

The system users are grouped into three sets:

  • u: user, the owner of the file
  • g: group users
  • o: other users

Files protections are:

  • r: read permission
  • w: write permission
  • x: execution permission

Directories protections are:

  • x: directory crossing
  • r: files list
  • w: files creation and/or deletion

Files and Directories protection Commands

Change the group of the files:

chgrp [-R] group file1 file2 ...

Change the owner (and potentially the group) of the files:

chown [-R] user[:group] file1 file2 ...

Change the protection of the files:

chmod [-R] protection file1 file2 ...
  • protection is the sum of: 4 for r, 2 for w and 1 for x respectively for user, group and others

Examples:

  • chmod 777 file: user has rwx access to the file, group has rwx access and also others (-rwxrwxrwx)
  • chmod 664 file: user has rw- access to the file, group has rw- access and others has r– access (-rw-rw-r–)
  • chmod 754 file: user has rwx access to the file, group has r-x access and others has r– access (-rwxr-xr–)

Options:

  • -R: recursive, the command works recursively for each subdirectory

The ls Command

ls [-options] [files]
  • list directory contents
  • Options are:
    • -a: all, list also hidden files
    • -l: long, long format output
    • -r: reverse, reverse order of output
    • -t: time, files are sorted by modification time
    • -R: recursive, the command works recursively for each subdirectory

ls command example

user@pc:~$ ls -al ~/tmp
 
drwxr-xr-x  3 user user  4096 2009-10-13 15:10 ./
drwxr-xr-x 64 user user 12288 2009-10-13 15:10 ../
drwxr-xr-x  2 user user  4096 2009-10-13 15:10 adir/
-rw-r--r--  1 user user    29 2009-10-13 15:10 afile
lrwxrwxrwx  1 user user     5 2009-10-13 15:24 alink -> afile

This command lists the tmp directory which is in the home directory showing also the hidden files with a long output format:

  • the first character is the file type (- standard file, d directory, l link)
  • rwxr-xr-x are the protections
  • the number of the second column is 1 for files and the number of file contained in the directory for directories
  • the first user is the owner, the second is the group
  • the second number is the dimension in byte
  • date and hour are the creation time
  • the last is the filename, in the case of soft link the linked file is reported

Viewing a textual file

Using visual editor like gedit, vim (or its oldest version vi) or emacs

With cat command

cat file1 file2 ...
  • print in order file1 and file2 content

With tail command

tail [-n number] file1 file2 ...
  • print the last number rows of file1 and file2 (default is 10 rows)
  • with +number prints the rows from number to the end of file1 and file2 (ex. tail -n +15)

With head command

head [-n number] file1 file2 ...}
  • like tail, but it prints the first number rows of file1 and file2

With more command

more file
  • prints a file page by page. Use space to go to the next page

With diff command

diff file1 file2
  • displays differences between file1 and file2

The find command

find <dir> [-opt]: it finds files present in the dir directory and in each subdirectory

Options are:

  • -name pattern: search pattern. Pattern can be a filename or a part of filename using the symbol \* to denote the rest of the file
  • examples of pattern:
    • abc: the file abc is searched
    • \*ab: files that end with ab are searched
    • bc\*: files that begin with bc are searched
    • \*dd\*: files that contain dd are searched
  • -type [b c d l]: b=block file, c=character file, d=directory, l=link
  • -exec command \;: for each file found executes command
  • -exec command \{} \;: for each file found executes command file_found. \{} is the file found.

The grep command

grep [-opt] pattern file1 file2 ...

The grep command prints lines matching a pattern found in file1, file2

Its options are:

  • -c: prints the number of matching lines
  • -i: case insensitive
  • -l: prints only names of files containing the pattern
  • -n: prints line number of matched lines
  • -v: prints only lines that non match pattern

pattern can be a regular expression:

  • .: represents any character
  • ^: the begin of the row
  • $: the end of the row
  • *: zero or more repetitions
  • +: one or more repetitions

The tar command

tar [options] tarfile [file1 file2 ...] [dir1 dir2 ...]

The tar command is used for the compression or decompression of files and directories

Compression:

  • c: creation of a new tar file
  • f: file, must be present both in the compression phase, both in the decompression phase
  • v: verbose mode
  • z: zipped, the file is compressed

Example:

tar cvzf /tmp/file.tgz /home/user/Documents /home/user/a.txt

it creates a verbose, zipped file. The Documents directory and the file a.txt are compressed in the archive /tmp/file.tgz.

Decompression:

  • x: extraction of files from an archive
  • t: test the content of an archive

Example 1:

tar tvzf /tmp/file.tgz

it tests a verbose zipped file, i.e., it tests the archive previously compressed

Example 2:

tar xvzf /tmp/file.tgz

it extracts a verbose, zipped file, i.e., it extracts in the current directory the archive previously compressed

The sort command

sort [-opt] file1 file2 ...

it sorts file1, file2, …

Options are:

  • -b: ignores blanks spaces at the beginning of a line
  • -f: case insensitive
  • -r: reverses the result
  • -n: numeric sort (for numbers)

The bash shell

  • A shell is a textual interface between the user and the operating system
  • The shell is not part of the Linux kernel, it is a normal userspace program
  • Since the shell is a normal process, different shells has been developed. The most commons are Bourne shell (sh}), C-shell (csh), Tahoe C-shell (tcsh) and Bourne again shell (bash)

The focus of this document is on bash

Each shell, when opened, search a configuration script in the home directory and run it

  • it can be used to customize the shell behavior or to initialize environment variables
  • in bash this file is .bashrc (it is located in the home directory and can be edited)

Completion and History

  • Using TAB the shell automatically complete file names (using the file present in the actual directory or in the directories listed in the environment variable $PATH).
  • ↓ or ↑: use arrows to scroll commands previously performed
  • use the history command to show the history buffer that lists the last executed commands associating them with a number
  • !n executes a command present in the buffer identified with the number n
  • !$ is the last parameter of the command previously performed
  • !* are all the parameter of the command previously performed
  • !string perform the last command that begin with string

Pipe

command1 | command2

A pipe is a connection between the first command stdout and the second command stdin

This linking is performed by the operator |

Examples:

  • ls | head -n 5
  • cat file | tail -n 25 that does the same thing of tail -n 25 file
  • cat file | sort -n -u | head -n 5

Process Management

  • Batch execution: comand1; command2, command1 is executed. When command1 is finished command2 is executed.
  • Concurrent execution: command1 \&; command2, command1 and command2 are executed concurrently
    • command1 is executed in background (symbol &).
    • it returns immediately the control to the shell that executes command2.

Summarizing:

  • normally a command is run foreground (fg)
  • with the symbol & after the command it can be run background (bg)
  • A program that run fg can be suspended pressing CTRL-Z
  • A suspended program can be restarted foreground using the command fg or background using the command bg

In the following figure the process states and the transitions between them are illustrated:

Scripts - Shell commands in a File

A sequence of commands can be written in a file and they can be executed directly calling it

It can be executed indirectly:

source <sciptfile> <args>

It can be executed directly:

  • the script file must have the execution permissions
  • the first row must begin with #! and with the absolute path of the shell used (example: \#!/bin/tcsh)
  • to run the script type ./scriptfile in the shell prompt

Shell Variables

variableName=value
  • instantiates a variable with the name variableName, setting it to value
  • e.g., a=10, b=“pippo”
$variableName
  • used to access the variable
  • to print a variable the command echo $variableName can be used
  • to print the value of variable b for example:
user@pc:~$ echo $b
pippo

Shell Environment Variables

  • Environment variables are written in upper-case and are used for the shell configuration
  • env: displays shell environment variables
  • printenv var: prints the value of var shell environment variables
  • export var: makes the variable var available to all the processes launched with the shell

Useful Environment Variables

  • HOME: the home directory
  • PATH: directory where the shell finds shell commands
  • SHELL: the used shell
  • LD_LIBRARY_PATH: it shows the directories where shared libraries are located

Example: add to the environment variable PATH the directory /home/user/bin:

export PATH=\$PATH:/home/user/bin

References

  1. Slides form which this page is derived: bash_shell_intro.pdf (Screen version), bash_shell_intro_printable.pdf (Printable version)
/membri/wiki/data/pages/cs/bash_shell.txt · Last modified: 2014/05/16 08:37 by Z
 
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